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21 files changed, 182 insertions, 133 deletions

diff --git a/files/ru/learn/css/building_blocks/каскад_задачи/index.html b/files/ru/learn/css/building_blocks/cascade_tasks/index.html
index b6524f9ed3..b6524f9ed3 100644
--- a/files/ru/learn/css/building_blocks/каскад_задачи/index.html
+++ b/files/ru/learn/css/building_blocks/cascade_tasks/index.html
diff --git a/files/ru/learn/css/introduction_to_css/ponimanie_osnov_css/index.html b/files/ru/learn/css/building_blocks/fundamental_css_comprehension/index.html
index 9009c684d8..9009c684d8 100644
--- a/files/ru/learn/css/introduction_to_css/ponimanie_osnov_css/index.html
+++ b/files/ru/learn/css/building_blocks/fundamental_css_comprehension/index.html
diff --git a/files/ru/learn/css/building_blocks/селекторы/attribute_selectors/index.html b/files/ru/learn/css/building_blocks/selectors/attribute_selectors/index.html
index 9a6a2c4c07..9a6a2c4c07 100644
--- a/files/ru/learn/css/building_blocks/селекторы/attribute_selectors/index.html
+++ b/files/ru/learn/css/building_blocks/selectors/attribute_selectors/index.html
diff --git a/files/ru/learn/css/building_blocks/селекторы/combinators/index.html b/files/ru/learn/css/building_blocks/selectors/combinators/index.html
index 7a076e05a8..7a076e05a8 100644
--- a/files/ru/learn/css/building_blocks/селекторы/combinators/index.html
+++ b/files/ru/learn/css/building_blocks/selectors/combinators/index.html
diff --git a/files/ru/learn/css/building_blocks/селекторы/index.html b/files/ru/learn/css/building_blocks/selectors/index.html
index 3819af4207..3819af4207 100644
--- a/files/ru/learn/css/building_blocks/селекторы/index.html
+++ b/files/ru/learn/css/building_blocks/selectors/index.html
diff --git a/files/ru/learn/css/building_blocks/селекторы/pseudo-classes_and_pseudo-elements/index.html b/files/ru/learn/css/building_blocks/selectors/pseudo-classes_and_pseudo-elements/index.html
index 4fe67b8adb..4fe67b8adb 100644
--- a/files/ru/learn/css/building_blocks/селекторы/pseudo-classes_and_pseudo-elements/index.html
+++ b/files/ru/learn/css/building_blocks/selectors/pseudo-classes_and_pseudo-elements/index.html
diff --git a/files/ru/learn/css/building_blocks/селекторы/селекторы_задачи/index.html b/files/ru/learn/css/building_blocks/selectors/selectors_tasks/index.html
index b8f36063c2..b8f36063c2 100644
--- a/files/ru/learn/css/building_blocks/селекторы/селекторы_задачи/index.html
+++ b/files/ru/learn/css/building_blocks/selectors/selectors_tasks/index.html
diff --git a/files/ru/learn/css/building_blocks/селекторы/type_class_and_id_selectors/index.html b/files/ru/learn/css/building_blocks/selectors/type_class_and_id_selectors/index.html
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--- a/files/ru/learn/css/building_blocks/селекторы/type_class_and_id_selectors/index.html
+++ b/files/ru/learn/css/building_blocks/selectors/type_class_and_id_selectors/index.html
diff --git a/files/ru/learn/css/css_layout/навыки_multicol/index.html b/files/ru/learn/css/css_layout/multicol_skills/index.html
index c549f1210b..c549f1210b 100644
--- a/files/ru/learn/css/css_layout/навыки_multicol/index.html
+++ b/files/ru/learn/css/css_layout/multicol_skills/index.html
diff --git a/files/ru/learn/css/css_layout/макет_с_несколькими_столбцами/index.html b/files/ru/learn/css/css_layout/multiple-column_layout/index.html
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--- a/files/ru/learn/css/css_layout/макет_с_несколькими_столбцами/index.html
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diff --git a/files/ru/learn/css/css_layout/нормальный_поток/index.html b/files/ru/learn/css/css_layout/normal_flow/index.html
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--- a/files/ru/learn/css/css_layout/нормальный_поток/index.html
+++ b/files/ru/learn/css/css_layout/normal_flow/index.html
diff --git a/files/ru/learn/css/css_layout/навыки_позиционирования/index.html b/files/ru/learn/css/css_layout/position_skills/index.html
index f63a3a3a94..f63a3a3a94 100644
--- a/files/ru/learn/css/css_layout/навыки_позиционирования/index.html
+++ b/files/ru/learn/css/css_layout/position_skills/index.html
diff --git a/files/ru/learn/css/css_layout/отзывчивый_дизайн/index.html b/files/ru/learn/css/css_layout/responsive_design/index.html
index 978b4e43dc..978b4e43dc 100644
--- a/files/ru/learn/css/css_layout/отзывчивый_дизайн/index.html
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diff --git a/files/ru/learn/css/css_properties/index.html b/files/ru/learn/css/css_properties/index.html
deleted file mode 100644
index a6d9e5d116..0000000000
--- a/files/ru/learn/css/css_properties/index.html
+++ /dev/null
@@ -1,133 +0,0 @@
----
-title: 'CSS properties: what they are and how to use them'
-slug: Learn/CSS/CSS_properties
-translation_of: Learn/CSS/Building_blocks/Selectors
-translation_of_original: Learn/CSS/CSS_properties
----
-<div class="summary">
-<p>{{Glossary("CSS")}} определяет как должна выглядеть вебстраница. Он использует предопределенные правила вместе с селекторами и свойствами для применения стилей к элементам HTML или группам элементов.</p>
-</div>
-
-<table class="learn-box standard-table">
- <tbody>
-  <tr>
-   <th scope="row">Prerequisites:</th>
-   <td>Basics of {{Glossary("HTML")}}, <a href="https://developer.mozilla.org/en-US/Learn/HTML/HTML_tags">HTML elements</a>, and <a href="https://developer.mozilla.org/en-US/Learn/CSS/Using_CSS_in_a_web_page#The_link_tag">how to link HTML documents to CSS stylesheets</a>.</td>
-  </tr>
-  <tr>
-   <th scope="row">Objective:</th>
-   <td>Learn about different CSS selectors and properties enough to style a simple webpage.</td>
-  </tr>
- </tbody>
-</table>
-
-<h2 id="Summary">Summary</h2>
-
-<p>Разделение содержимого и стиля делает Веб разработку намного быстрее и проще. Когда вы определяете только стуктуру документа в вашем HTML файле и храните всю информацию о стиле в отдельном файле (называемом stylesheet), вы можете обновлять стили нескольких документов одновременно (а так же экономить ресурсы компьютера).</p>
-
-<p>CSS syntax consists of easy-to-use, intuitive keywords.</p>
-
-<pre class="brush: css">p {
-   font-family: "Times New Roman", georgia, sans-serif;
-   font-size: 24px;
-}</pre>
-
-<p>In the example above, <code>p</code> is a selector that applies styles to all the <code>{{HTMLElement("p")}}</code> elements at once. The CSS properties <code>font-family</code> and <code>font-size</code> are enclosed within curly braces and the corresponding values, right after the colon, determine the styles.</p>
-
-<p>There are more than <a href="/en-US/docs/Web/CSS/Reference">250 properties</a> you can apply to your document. From text to layout, (almost) anything is possible.</p>
-
-<h2 id="Active_Learning">Active Learning</h2>
-
-<p><em>There is no active learning available yet. <a href="/en-US/docs/MDN/Getting_started">Please, consider contributing</a>.</em></p>
-
-<h2 id="Deeper_dive">Deeper dive</h2>
-
-<p>If properties are fairly simple to use, selectors are another story. Okay, they aren't that hard, and mastering them unleashes the full potential of CSS. In the next examples, we will introduce the most common selectors.</p>
-
-<p>A CSS rule consists of selectors associated with properties. Selectors specify which elements will receive the properties laid down in the rule. Multiple rules can apply to the same element; the CSS cascade (which we'll discuss later on) determines which rule ends up taking effect in the case of conflicts. For now, just remember that the rule with the most <a href="/en-US/docs/Web/CSS/Specificity">specific selector</a> overrides the rules with more generic selectors.</p>
-
-<h3 id="The_element_selector">The element selector</h3>
-
-<p>Element selectors select HTML elements by element names only. Moreover, like all CSS selectors, you can apply a common set of properties to several elements at once.</p>
-
-<p>For our first example, let's assume the following HTML code fragment:</p>
-
-<pre class="brush: html">&lt;h1&gt;I'm an example&lt;/h1&gt;
-&lt;p&gt;In this example, I'm a paragraph&lt;/p&gt;
-&lt;p&gt;And I'm another paragraph&lt;/p&gt;
-</pre>
-
-<p>In the following CSS rule, the element selector <code>p</code> applies the given styles simultaneously to all the <code>{{HTMLElement("p")}}</code> elements of our HTML document, preventing extensive rewriting. We are using the {{cssxref("font-family")}} property (which defines the font in which text appears) and the {{cssxref("font-size")}} (which defines text size).</p>
-
-<pre class="brush: css">p {
-  font-family: "Helvetica", Arial, sans-serif;
-  font-size  : 12px;
-}</pre>
-
-<p>The next CSS rule only applies to <code>{{HTMLElement("h1")}}</code> elements. We are using the {{cssxref("font-size")}} property to make our title twice the size of the body text, and the {{cssxref("font-weight")}} property to make the title bold.</p>
-
-<pre class="brush: css">h1 {
-  font-size  : 24px;
-  font-weight: bold;
-}</pre>
-
-<p>The following CSS rule applies the requisite styles to both <code>{{HTMLElement("h1")}}</code> and <code>{{HTMLElement("p")}}</code> elements, potentially removing even more duplication. (This use is called "group selector" or "chain selector". Notice the comma separating the selectors). Here we are using the {{cssxref("color")}} property to specify the same text color for both headings and paragraphs.</p>
-
-<pre class="brush: css">h1, p {
-  color: darkmagenta;
-}</pre>
-
-<p>Here is the result of all this code:</p>
-
-<p>{{ EmbedLiveSample('The_element_selector') }}</p>
-
-<h3 id="The_id_selector">The id selector</h3>
-
-<p>The <code>id</code><strong> </strong>attribute of a particular HTML element uniquely identifies that element. Hence, an id selector is used only when a set of style rules applies to a single element.</p>
-
-<p>For our next example, let's assume the following HTML code fragment:</p>
-
-<pre class="brush: html">&lt;p id="hello"&gt;Hello world!&lt;/p&gt; </pre>
-
-<p>The following CSS rule applies only to that unique identified element. To make a selector into an id selector, you must put a hash character (#) in front of the id name. We are using three properties: {{cssxref("text-align")}} to center the text within the paragraph {{cssxref("border")}} to add a thin line around the paragraph, and {{cssxref("padding")}} to add some extra inner-margin between the text and the border.</p>
-
-<pre class="brush: css">#hello {
-  text-align: center;
-  border    : 1px solid black;
-  padding   : 8px;
-}</pre>
-
-<p>And the result is the following:</p>
-
-<p>{{ EmbedLiveSample('The_id_selector') }}</p>
-
-<h3 id="The_class_selector">The class selector</h3>
-
-<p>Within HTML, the <code>class</code><strong> </strong>attribute lets you apply multiple identifiers to HTML elements. Those identifiers can be used with CSS to match groups of elements regardless of element name.</p>
-
-<p>For our next example, let's assume the following HTML code fragment:</p>
-
-<pre class="brush: html">&lt;h1 class="hello"&gt;Hey there!&lt;/h1&gt;
-&lt;p class="hello bye"&gt;Let's hang out together!&lt;/p&gt;
-&lt;p class="bye"&gt;And walk over the mountain&lt;/p&gt;
-</pre>
-
-<p>Let's apply a CSS rule for all elements with the class <code>hello</code>. To make the selector into a class selector, put a period/full stop before the class name. We use the {{cssxref("font-style")}} property to italicize the text.</p>
-
-<pre class="brush: css">.hello {
-  font-style: italic;
-}</pre>
-
-<p>And another one for all elements with the class <code>bye</code>. Here we are using the {{cssxref("text-decoration")}} property to draw a line through the text.</p>
-
-<pre class="brush: css">.bye {
-  text-decoration: line-through;
-}</pre>
-
-<p>Here's what happened:</p>
-
-<p>{{ EmbedLiveSample('The_class_selector') }}</p>
-
-<h2 id="Next_step">Next step</h2>
-
-<p>So we've gone over the basics to get started with CSS. You can <a href="/en-US/docs/Learn/CSS/Basic_text_styling_in_CSS">learn more about text styling</a> or start exploring<a href="/en-US/docs/Web/CSS/Tutorials"> our CSS Tutorials</a> right away.</p>
diff --git a/files/ru/learn/css/first_steps/как_структурирован_css/index.html b/files/ru/learn/css/first_steps/how_css_is_structured/index.html
index d2c60edcfb..d2c60edcfb 100644
--- a/files/ru/learn/css/first_steps/как_структурирован_css/index.html
+++ b/files/ru/learn/css/first_steps/how_css_is_structured/index.html
diff --git a/files/ru/learn/css/first_steps/что_такое_css/index.html b/files/ru/learn/css/first_steps/what_is_css/index.html
index 6b5bdf8924..6b5bdf8924 100644
--- a/files/ru/learn/css/first_steps/что_такое_css/index.html
+++ b/files/ru/learn/css/first_steps/what_is_css/index.html
diff --git a/files/ru/learn/css/howto/css_faq/index.html b/files/ru/learn/css/howto/css_faq/index.html
new file mode 100644
index 0000000000..cecfb92b82
--- /dev/null
+++ b/files/ru/learn/css/howto/css_faq/index.html
@@ -0,0 +1,182 @@
+---
+title: Common CSS questions
+slug: Web/CSS/Common_CSS_Questions
+translation_of: Learn/CSS/Howto/CSS_FAQ
+---
+<h2 id="Why_doesn't_my_CSS_which_is_valid_render_correctly">Why doesn't my CSS, which is valid, render correctly?</h2>
+
+<p>Браузер использует декларацию <code>DOCTYPE</code> чтобы выбрать, как именно отображать документ - в форме, более совместимой с современными стандартами или в форме,  которую будут поддерживать старые браузеры. Правильное использование декларациии <code>DOCTYPE</code> в начале вашего HTML кода повлияет на совместимость с современными стандартами веб браузеров.</p>
+
+<p>У современных браузеров есть два режима отображения веб-страниц:</p>
+
+<ul>
+ <li><em>Индивидуальный</em>: его также называют backwards-compatibility mode, даёт возможность устаревшим страницам отображаться так, как планировал автор, следуя уже не стандартным правилам отображения, которые использовались ещё старыми браузерами. Документы  с неполной, некорректной или отстутвующей <code>DOCTYPE</code> декларацией или с тем видом <code>DOCTYPE</code>, который использовался до 2001 года, будет отображён в индивидуальном режиме.</li>
+ <li><em>Стандартный</em>: в этом режиме браузер старается строго следовать стандартам W3C. Ожидается, что современные HTML страницы разработаны для браузеров, следуемых стандартам, и в результате, страницы с современным  <code>DOCTYPE</code> отображаются уже в стандартом режиме.</li>
+</ul>
+
+<p>Gecko-based browsers, have a third <em><a href="/en-US/docs/Gecko's_&quot;Almost_Standards&quot;_Mode" title="Gecko's_&quot;Almost_Standards&quot;_Mode">Almost Standards Mode</a></em> that has only a few minor quirks.</p>
+
+<p>This is a list of the most commonly used <code>DOCTYPE</code> declarations that will trigger Standards or Almost Standards mode:</p>
+
+<pre>&lt;!DOCTYPE html&gt; /* This is the HTML5 doctype. Given that each modern browser uses an HTML5
+                   parser, this is the recommended doctype */
+
+&lt;!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"
+"http://www.w3.org/TR/html4/loose.dtd"&gt;
+
+&lt;!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
+"http://www.w3.org/TR/html4/strict.dtd"&gt;
+
+&lt;!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
+"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"&gt;
+
+&lt;!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
+"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"&gt;
+</pre>
+
+<h2 id="My_CSS_is_valid.2C_but_not_correctly_rendered" name="My_CSS_is_valid.2C_but_not_correctly_rendered">Why doesn't my CSS, which is valid, render at all?</h2>
+
+<p>To be applied, a CSS stylesheet must be served with a <code>text/css</code> MIME type. If the Web server doesn't serve it with this type, it won't be applied.</p>
+
+<h2 id="Difference_between_id_and_class" name="Difference_between_id_and_class">What is the difference between <code>id</code> and <code>class</code>?</h2>
+
+<p>HTML elements can have an id and/or class attribute. The id attribute assigns a name to the element it is applied to, and for valid markup, there can be only one element with that name. The class attribute assigns a class name to the element, and that name can be used on many elements within the page. CSS allows you to apply styles to particular id and/or class names.<br>
+ <br>
+ Use an id-specific style when you want to restrict the applied styling rules to one specific block or element. This style will only be used by the element with that particular id.<br>
+ <br>
+ Use a class-specific style when you want to apply the styling rules to many blocks and elements within the page.</p>
+
+<p>Stylesheets with fewer rules are usually more performant. It is therefore recommended to use classes as much as possible, and to reserve the use of id for specific uses (like to connect label and form elements or for styling elements that must be semantically unique).</p>
+
+<p>See <a href="/en-US/docs/CSS/Getting_Started/Selectors" title="CSS/Getting_Started/Selectors"> CSS selectors</a></p>
+
+<h2 id="Restoring_the_default_property_value" name="Restoring_the_default_property_value">How do I restore the default value of a property?</h2>
+
+<p>Initially CSS didn't provide a "default" keyword and the only way to restore the default value of a property is to explicitly re-declare that property.</p>
+
+<p>This has changed with CSS 2; the keyword <a href="/es/CSS/initial" title="initial">initial</a> is now a valid value for a CSS property. It resets it to its default value, which is defined in the CSS specification of the given property.</p>
+
+<h2 id="Derived_styles" name="Derived_styles">How do I derive one style from another?</h2>
+
+<p>CSS does not allow one style to be defined in terms of another. (See <a href="http://archivist.incutio.com/viewlist/css-discuss/2685">Eric Meyer's note about the Working Group's stance</a>). However, assigning multiple classes to a single element can provide the same effect.</p>
+
+<h2 id="Assigning_multiple_classes" name="Assigning_multiple_classes">How do I assign multiple classes to an element?</h2>
+
+<p>HTML elements can be assigned multiple classes by listing the classes in the <code>class</code> attribute, with a blank space to separate them.</p>
+
+<pre>&lt;style type="text/css"&gt;
+.news { background: black; color: white; }
+.today { font-weight: bold; }
+&lt;/style&gt;
+
+&lt;div class="news today"&gt;
+... content of today's news ...
+&lt;/div&gt;
+</pre>
+
+<p>If the same property is declared in both rules, the conflict is resolved first through specificity, then according to the order of the CSS declarations. The order of classes in the <code>class</code> attribute is not relevant.</p>
+
+<h2 id="Style_rules_that_don.27t_work" name="Style_rules_that_don.27t_work">Why don't my style rules work properly?</h2>
+
+<p>Style rules that are syntactically correct may not apply in certain situations. You can use <a href="/en-US/docs/DOM_Inspector" title="DOM_Inspector">DOM Inspector</a>'s <em>CSS Style Rules</em> view to debug problems of this kind, but the most frequent instances of ignored style rules are listed below.</p>
+
+<h3 id="HTML_elements_hierarchy" name="HTML_elements_hierarchy">HTML elements hierarchy</h3>
+
+<p>The way CSS styles are applied to HTML elements depends also on the elements hierarchy. It is important to remember that a rule applied to a descendent overrides the style of the parent, in spite of any specificity or priority of CSS rules.</p>
+
+<pre>.news { color: black; }
+.corpName { font-weight: bold; color: red; }
+
+&lt;!-- news item text is black, but corporate name is red and in bold --&gt;
+&lt;div class="news"&gt;
+   (Reuters) &lt;span class="corpName"&gt;General Electric&lt;/span&gt; (GE.NYS) announced on Thursday...
+&lt;/div&gt;
+</pre>
+
+<p>In case of complex HTML hierarchies, if a rule seems to be ignored, check if the element is inside another element with a different style.</p>
+
+<h3 id="Explicitly_re-defined_style_rule" name="Explicitly_re-defined_style_rule">Explicitly re-defined style rule</h3>
+
+<p>In CSS stylesheets, order <strong>is</strong> important. If you define a rule and then you re-define the same rule, the last definition is used.</p>
+
+<pre>#stockTicker { font-weight: bold; }
+.stockSymbol { color: red; }
+/*  other rules             */
+/*  other rules             */
+/*  other rules             */
+.stockSymbol { font-weight: normal; }
+
+&lt;!-- most text is in bold, except "GE", which is red and not bold --&gt;
+&lt;div id="stockTicker"&gt;
+   NYS: &lt;span class="stockSymbol"&gt;GE&lt;/span&gt; +1.0 ...
+&lt;/div&gt;
+</pre>
+
+<p>To avoid this kind of error, try to define rules only once for a certain selector, and group all rules belonging to that selector.</p>
+
+<h3 id="Use_of_a_shorthand_property" name="Use_of_a_shorthand_property">Use of a shorthand property</h3>
+
+<p>Using shorthand properties for defining style rules is good because it uses a very compact syntax. Using shorthand with only some attributes is possible and correct, but it must be remembered that undeclared attributes are automatically reset to default. This means that a previous rule for a single attribute could be implicitly overridden.</p>
+
+<pre>#stockTicker { font-size: 12px; font-family: Verdana; font-weight: bold; }
+.stockSymbol { font: 14px Arial; color: red; }
+
+&lt;div id="stockTicker"&gt;
+   NYS: &lt;span class="stockSymbol"&gt;GE&lt;/span&gt; +1.0 ...
+&lt;/div&gt;
+</pre>
+
+<p>In the previous example the problem occurred on rules belonging to different elements, but it could happen also for the same element, because rule order <strong>is</strong> important.</p>
+
+<pre>#stockTicker {
+   font-weight: bold;
+   font: 12px Verdana;  /* font-weight is now normal */
+}
+</pre>
+
+<h3 id="Use_of_the_.2A_selector" name="Use_of_the_.2A_selector">Use of the <code>*</code> selector</h3>
+
+<p>The <code>*</code> wildcard selector refers to any element, and it has to be used with particular care.</p>
+
+<pre>body * { font-weight: normal; }
+#stockTicker { font: 12px Verdana; }
+.corpName { font-weight: bold; }
+.stockUp { color: red; }
+
+&lt;div id="section"&gt;
+   NYS: &lt;span class="corpName"&gt;&lt;span class="stockUp"&gt;GE&lt;/span&gt;&lt;/span&gt; +1.0 ...
+&lt;/div&gt;
+</pre>
+
+<p>In this example the <code>body *</code> selector applies the rule to all elements inside body, at any hierarchy level, including the .stockUp class. So <code>font-weight: bold;</code> applied to the .corpName class is overridden by <code>font-weight: normal;</code> applied to all elements in the body.</p>
+
+<p>The use of the * selector should be minimized as it is a slow selector, especially when not used as the first element of a selector. Its use should be avoided as much as possible.</p>
+
+<h3 id="Specificity_in_CSS" name="Specificity_in_CSS">Specificity in CSS</h3>
+
+<p>When multiples rules apply to a certain element, the rule chosen depends on its style <a href="/en-US/docs/CSS/Specificity" title="Specificity">specificity</a>. Inline style (in HTML <code>style</code> attributes) comes first, followed by ID selectors, then class selectors and eventually element-name selectors.</p>
+
+<pre>div { color: black; }
+#orange { color: orange; }
+.green { color: green; }
+
+&lt;div id="orange" class="green" style="color: red;"&gt;This is red&lt;/div&gt;
+</pre>
+
+<p>The rules are more complicated when the selector has multiple parts. More detailed information about how selector specificity is calculated can be found in the <a href="http://www.w3.org/TR/CSS21/cascade.html#specificity">CSS 2.1 Specification chapter 6.4.3</a>.</p>
+
+<h2 id="What_do_the_-moz-.2A_properties_do.3F" name="What_do_the_-moz-.2A_properties_do.3F">What do the -moz-*, -ms-*, -webkit-*, -o-* and -khtml-* properties do?</h2>
+
+<p>These properties, called <em>prefixed properties</em>, are extensions to the CSS standard. They are used to use experimental and non-standard features without polluting the regular namespace, preventing future incompatibilities to arise when the standard is extended.</p>
+
+<p>The use of such properties on production websites is not recommended. If nevertheless needed, you are hinted to make a plan for the website evolution: these prefixed properties can be modified or even suppressed when the standard evolves.</p>
+
+<p>Please see the <a href="/en-US/docs/CSS/CSS_Reference/Mozilla_Extensions" title="CSS Reference/Mozilla Extensions">Mozilla CSS Extensions</a> page for more information on the Mozilla-prefixed CSS properties.</p>
+
+<h2 id="How_does_z-index_relate_to_positioning">How does z-index relate to positioning?</h2>
+
+<p>The z-index property specifies the stack order of elements.</p>
+
+<p>An element with a higher z-index/stack order is always in front of an element with a lower z-index/stack order.</p>
+
+<p>Z-index will only work on elements that have a specified position (<code>position:absolute</code>, <code>position:relative</code>, or <code>position:fixed</code>).</p>
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